The present invention generally relates to radio base stations, and more particularly to a radio base station for a CDMA (Code Division Multiple Access) mobile communication system, which uses the CDMA transmission technique to enable transmission at a plurality of different bit rates.
According to the CDMA transmission technique, a signal is first subjected to a spreading process using an allocated code and is then transmitted. Unlike the FDMA transmission technique and the TDMA transmission technique, the CDMA transmission technique performs the communication while tolerating interferences from other radio channels up to a predetermined tolerable target line quality value. The line quality is determined by a ratio of xe2x80x9c(Eb/No)xe2x80x9d, where Eb is the signal power per a single information bit and No is the noise power.
The noise power No mainly includes interference noise from other mobile stations MS, which perform radio communications with the same radio base station BS. This interference noise is particularly dependent upon the number of radio channels simultaneously set in a direction (reverse link) from the mobile station MS to the radio base station BS.
Accordingly, in order to maintain a particular line quality, the number of mobile stations MS simultaneously communicating with the same radio base station BS must be limited. Even if a sufficient margin exists in the hardware when carrying out the transmission process, the line quality may deteriorate and a failure such as a disconnection may occur if calls exceeding the above described limit are accepted.
For this reason, a maximum radio channel number Nrmax which can be allocated is theoretically calculated and set based on experience. This maximum radio channel number Nrmax is set with respect to the radio bases station BS. Further, the allocation of a radio channel is rejected with respect to a new call generated after allocating the maximum radio channel number Nrmax with respect to the calls.
On the other hand, in a mobile communication system using the CDMA transmission technique, there are additional demands to provide information transmission service beyond a basic bit rate of 8 kb/s. For example, an information transmission service at a higher bit rate such as 13 kb/s may be required to improve communication quality or to realize high-speed data communications.
When providing information transmission services at a plurality of different bit rates, the bit rate to be used is arranged between the radio base station BS and the mobile station MS when setting up the call. Therefore, the communication is made at the bit rate determined by this arrangement.
For example, when information transmission services using two different bit rates are provided in the mobile communication system, the transmission cannot be made at the bit rate of 13 kb/s if either the radio base station BS or the mobile station MS cannot support the information transmission service at the bit rate of 13 kb/s. Thus, the transmission in this case is made at the basic bit rate of 8 kb/s. On the other hand, if both the radio base station BS and the mobile station MS both support the information transmission service at the bit rate of 13 kb/s, the transmission can be made at the bit rate of 13 kb/s.
However, since a transmission output of the mobile station MS is suppressed to less than a predetermined value, the signal power Eb per one information bit becomes smaller as the bit rate becomes higher. For this reason, in order to maintain the same line quality (Eb/No) even when the transmission is made at the higher bit rate, it is necessary to reduce the interference noise of mobile stations MS that can be connected simultaneously to the radio base station BS. In other words, the maximum radio channel number Nrmax that can be allocated, must be small compared to the case where the transmission is made at the lower bit rate.
Accordingly, in a conventional mobile communication system in which a plurality of bit rates are used, the maximum radio channel number Nrmax that can be allocated is set by assuming a case where all of the radio channels are allocated to the calls requesting the higher bit rate. The allocation of a radio channel is rejected with respect to a call generated after allocating the maximum radio channel number Nrmax with respect to the calls.
However, when the maximum radio channel number Nrmax that can be allocated is set by assuming the case where all of the radio channels are allocated to the calls requesting the high bit rate and the allocation of the radio channel is rejected with respect to the call generated after allocating the maximum radio channel number Nrmax with respect to the calls, the utilization efficiency of the radio channels is low.
In other words, in actual practice, there are many cases where a large number of calls for information transmission service at the lower bit rate are included among the calls. In such cases, there is usually no immediate undesirable effect on the line quality even if the radio channels are newly allocated to the calls by accepting the number of calls exceeding the maximum radio channel number Nrmax that can be allocated by the radio base station BS.
Therefore, according to the above described method of setting the maximum radio channel number Nrmax which can be allocated, the resources of the radio channel are not utilized effectively to the limit. The number of mobile stations MS that can be accommodated by one radio base station BS is an important factor which directly affects the cost of the equipment per subscriber of the mobile station MS. The effective utilization of the resources of the radio channels can reduce the cost of the entire mobile communication system, which could greatly affect the rates or charges of the services.
Accordingly, it is an object of the present invention to provide a useful radio base station for a CDMA mobile communication system, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a radio base station for a CDMA mobile communication system which provides information transmission services at a plurality of different bit rates, wherein resources of radio channels are effectively utilized by efficiently allocating the radio channels so that a large number of calls can be accepted.
These and other objects are met by providing a radio base station in a CDMA mobile communication system for making a radio communication among mobile stations at a plurality of different bit rates. The base station according to the present invention including a storage part for storing a maximum number of radio channels that makes a signal-to-interference noise power ratio for each channel greater than a predetermined value. The maximum number of radio channels having a value which changes in steps depending on the bit rate used for a call presently made. A control part for reading the maximum value stored in the storage part and allocating radio channels to the mobile stations in a range which does not exceed the maximum value. According to the radio base station of the present invention, it is possible to increase the number of subscribers of the mobile stations that can be accommodated per sector or cell. In addition, it is possible to reduce undesirable instances where the line is busy due to traffic. Therefore, it is possible to increase the number of mobile station subscribers which can be accommodated at a lower cost, without the need to modify and increase the transmission equipment such as by increasing the number of radio frequencies.
The storage part may store a data table including combinations of limit values for the number of radio channels for each of the bit rates used. The control part may refer to the data table with respect to a radio channel allocation request and accept a call only when the number of radio channels already allocated is less than a limit value corresponding to the bit range of the call stored in the data table.
The storage part may also store a reference value, which is smaller than a maximum number of radio channels that corresponds to the signal-to-interference noise ratio being greater than the predetermined value when all calls which are made using the highest bit rate. In this case, the control part may allocate a radio channel only at a basic bit rate which is lower than the highest bit rate with respect to a call which is generated in a state where the number of radio channels allocated exceeds the reference value.
The control part may also allocate with respect to a mobile station, a radio channel at a lower bit rate than the bit rate requested by a radio channel allocation request. This allocation would apply to the case where the number of radio channels already allocated at the lower bit rate is less than the limit value stored in the data table and the number of radio channels already allocated at the bit rate requested by the ratio channel allocation request exceeds the limit value stored in the data table.
The storage part may also store a data table including combinations of limit values of the number of radio channels for each of the bit rates used. The control part may allocate with respect to a mobile station a radio channel at a lower bit rate than requested by a radio channel allocation request. This allocation would apply to the case where the number of radio channels already allocated is less than the limit value stored in the data table and the number of radio channels already allocated exceeds the limit value stored in the data table at the bit rate requested by the radio channel allocating request.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.